The present invention relates generally to batch ion implantation systems, and more particularly to an apparatus for the backside cooling of a wafer in a batch ion implanter and a seal apparatus therefor.
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in xe2x80x9cn typexe2x80x9d extrinsic material wafers, whereas if xe2x80x9cp typexe2x80x9d extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
Ion implanters may be separated into two different categories. The first category includes serial ion implanters, in which semiconductor wafers or other workpieces are completely implanted with ions in serial fashion. This type of implanter includes a single workpiece mount adapted to hold or support the workpiece being implanted. The second category of ion implanters includes batch implanters, wherein a plurality of wafers or other workpieces may be implanted with ions in a single batch. The workpieces being implanted are mounted in individual workpiece mounts in a rotatable process disk. The workpiece mounts are typically located on individual pedestals extending outward from a center portion of the process disk at a slight angle so as to use centrifugal force to keep the workpieces seated in the mounts as the process disk is rotated in a controlled fashion via a drive motor. The ion source is located so as to present ions along a beam path offset from the rotational axis of the process disk, and thereby to implant ions onto the workpieces as they rotate into the beam path. This method of ion implantation is sometimes referred to as spinning disk ion implantation.
As ions are implanted in the workpieces, heat is generated therein, which may cause workpiece damage or other deleterious effects if not removed from the workpiece. Conventional batch ion implantation systems and apparatus remove heat from the process disk onto which the workpieces are mounted, using internal passages through which cooling fluid such as water is circulated. Heat is removed from the workpieces to the process disk through vulcanized rubber or RTV pads on which the workpieces are seated. The RTV pads provide some heat removal by transferring heat from the workpieces to the process disk. However, improved heat transfer is desirable, in order to minimize thermal damage to the workpieces. In addition, the relatively sticky RTV presently employed for heat sinking may cause other problems. In particular, the workpieces may stick to the RTV, making removal of the workpieces from the batch implanter difficult. Furthermore, particles tend to adhere to the RTV, which may be transferred to the workpieces causing undesirable contamination thereof. Moreover, the RTV pads may cause capacitive charging of the workpieces. Thus, there is an unresolved need for improved batch ion implantation systems and apparatus which eliminate or minimize the problems associated with conventional RTV workpiece heat sink pads, and which provide for improved heat transfer away from the workpieces.
The present invention is directed to a system and apparatus which provides improved heat transfer from workpieces in a batch ion implantation operation, while eliminating RTV pads heretofore employed and the problems associated therewith. The invention provides an apparatus by which a cooling gas is supplied from a stationary source to the back side of the workpieces being implanted in a rotating or spinning batch implanter process disk. The provision of the cooling gas allows for improved heat transfer from the workpieces to the process disk, which may be advantageously combined with circulation of cooling fluid through passages in the process disk to remove heat therefrom. The invention further includes the use of a rotary feedthrough in order to transfer the cooling gas from a stationary housing to a gas chamber in a rotating shaft which spins the batch implanter process disk. In addition, a seal apparatus is provided which seals the cooling gas applied to the back sides of the workpieces from the vacuum in which the front sides of the workpieces are implanted. The invention thus improves heat transfer and reduces or eliminates particulate transfer, wafer sticking, and wafer capacitive charging associated with conventional batch implanters.
In accordance with one aspect of the present invention, there is provided an ion implantation system comprising a housing with an outer wall having an outer surface and an inner surface defining an interior cavity, and a first gas chamber extending through the outer wall between a first gas inlet opening in the outer surface of the outer wall and an outlet opening in the inner surface. A shaft is rotatably mounted in the interior cavity of the housing for rotation about an axis, which includes an outer surface extending axially between a first end and a second end. The shaft includes a second gas chamber extending there through between a second gas inlet opening through the outer surface of the shaft and the second end thereof. A process disk is mounted onto or otherwise operatively engaged with the second end of the shaft for rotation about the axis, including a third gas chamber in fluid communication with the second gas chamber of the shaft.
The process disk comprises one or more pedestals extending laterally outwardly from a center portion, wherein the pedestals may each include a workpiece mount radially disposed from the axis and adapted to support a workpiece thereon. The pedestals comprise a gas feed port in the corresponding workpiece mount, wherein the third gas chamber provides fluid communication between the second gas chamber in the shaft and the gas feed ports in the workpiece mounts. The system may further comprise a drive, such as a motor, adapted to provide rotation of the shaft with respect to the housing, and a cooling gas source adapted to provide gas to the gas inlet opening in the housing, whereby cooling gas may be provided to the back sides of the workpieces via the first, second, and third gas chambers in order to remove heat from the workpieces as ions are implanted thereon.
A rotary feedthrough is included in the system providing fluid communication between the outlet opening of the housing and the second gas inlet of the shaft. This feedthrough may comprise, for example, a magnetic seal adapted to provide a seal between the interior cavity of the housing and the first and second gas chambers. In this regard, the magnetic seal may comprise a cylindrical member having an inner surface disposed radially outwardly of and encircling the shaft to form a gap there between, an outer surface mounted on the inner surface of the housing, a magnet forming a magnetic circuit with the shaft, and a magnetic fluid filling a portion of the gap between the cylindrical member inner surface and the shaft, wherein the magnetic fluid provides the seal between the interior cavity of the housing and the first and second gas chambers. Thus configured, cooling gas may be provided from the cooling gas source to the workpiece at the gas feed port of the workpiece mount via the first, second, and third gas chambers in order to remove heat from the workpiece.
In application with an ion implanter which operates in a vacuum, the magnetic seal thus provides a seal between the vacuum and the cooling gas, as well as between the cooling gas (which may be pressurized), and the ambient. This dual seal arrangement facilitates the application of pressurized cooling gas to the back sides of the spinning wafer workpieces in a batch ion implanter which is free of impurities. In this regard, the interior cavity of the stationary housing may comprise a first cavity end proximate the first end of the shaft and a second cavity end disposed near the second end of the shaft, wherein the first cavity end may be at ambient pressure, and the second cavity end may be in the evacuated implantation process chamber. Accordingly, the cylindrical feedthrough member extends axially between a first end disposed toward the first end of the shaft and a second end disposed toward the second end of the shaft. The rotary feedthrough may further comprise a first seal portion near the first end of the cylindrical member and providing a seal between the first cavity end (e.g., ambient pressure) and the first and second gas chambers (e.g., pressurized cooling gas), and a second seal portion near the second end of the cylindrical member and providing a seal between the second cavity end (e.g., vacuum) and the first and second gas chambers (e.g., cooling gas).
In a dual seal magnetic feedthrough, for example, the first seal portion of the cylindrical member may comprise a first inner surface disposed radially outwardly of and encircling the shaft to form a first gap there between, which is axially disposed between the first end of the cylindrical member and the first and second gas chambers, and a first outer surface mounted on the inner surface of the housing. A first magnet forms a first magnetic circuit with the shaft, and a first magnetic fluid fills a portion of the first gap between the cylindrical member inner surface and the shaft. Thus configured, the first magnetic fluid provides a reliable seal between the first cavity end and the first and second gas chambers (e.g., between the ambient and the cooling gas) which prevents escape of the cooling gas to the ambient environment, or of ambient gas to the cooling gas, and introduction of impurities into the cooling gas.
Similarly, the second seal portion may comprise a second inner surface disposed radially outwardly of and encircling the shaft to form a second gap there between, the second gap being axially disposed between the second end of the cylindrical member and the first and second gas chambers, and a second outer surface mounted on the inner surface of the housing. A second magnet forms a second magnetic circuit with the shaft, and a second magnetic fluid fills a portion of the second gap between the cylindrical member inner surface and the shaft. The second magnetic fluid thus provides a seal between the second cavity end and the first and second gas chambers (e.g., between the cooling gas and the vacuum).
According to another aspect of the invention, a workpiece support is provided for supporting at least one workpiece in a batch ion implantation system. The support comprises a housing with an outer wall having an outer surface and an inner surface defining an interior cavity, and a first gas chamber extending through the outer wall between a first gas inlet opening in the outer surface of the outer wall and an outlet opening in the inner surface, and a shaft rotatably mounted in the interior cavity of the housing for rotation about an axis and having an outer surface extending axially between a first end and a second end. The shaft includes a second gas chamber extending there through between a second gas inlet opening through the outer surface of the shaft and the second end. A drive may be included for providing rotation of the shaft with respect to the housing in order to effectuate spinning disk implantation in the batch implanter.
A process disk is mounted on the second end of the shaft for rotation about the axis, and includes a center portion located along the axis and one or more pedestals extending laterally outwardly from the center portion. The pedestals comprise a workpiece mount radially disposed from the axis and adapted to support a workpiece thereon, which in turn includes a gas feed port. The process disk further comprises a third gas chamber providing fluid communication between the second gas chamber in the shaft and the gas feed ports of the workpiece mounts. The workpiece support further includes a rotary feedthrough providing fluid communication between the outlet opening of the housing and the second gas inlet of the shaft, which may include a magnetic seal. The rotary feedthrough may advantageously provide first and second seals between the gas chambers and first and second ends of the housing cavity, whereby cooling gas passing there through is sealed from the outside ambient pressure as well as from the vacuum chamber in which the front surfaces of the workpieces are implanted with ions.
According to yet another aspect of the invention, there is provided a seal apparatus for sealing a front side of a wafer from cooling gas supplied to the back side thereof. The seal may be employed in a batch ion implanter with a rotatable process disk having at least one workpiece mount adapted to support the workpiece and to supply a cooling gas to the workpiece back side. The seal apparatus comprises a ring-shaped support member mounted on the workpiece mount, and a first seal member mounted on the support member having a first portion flexible with respect to the support member and adapted to engage a portion of the backside of the workpiece proximate the peripheral edge to seal the front side of the workpiece from the cooling gas.
The first portion of the first seal member may be flexible in a direction generally perpendicular to the plane of the workpiece. This allows the centrifugal force resulting from rotation of the workpiece in the process disk to deflect the first seal member around the peripheral edge of the workpiece to provide sealing engagement there between. Alternatively or in combination, a mechanical clamping arrangement may be used to provide flexure of the first seal member to effectuate a sealing engagement with a peripheral surface of the workpiece. The first portion of the first seal member may also be cantilevered radially outwardly with respect to the center of the ring-shaped support member so as to pivot about a circumferential axis in a direction generally perpendicular to the plane of the workpiece.
In addition, the first portion of the first seal member may include a rib extending toward and engaging the backside of the workpiece proximate the peripheral edge, which may have a v-shaped profile with a pointed edge extending toward and engaging the backside of the workpiece proximate the peripheral edge. The ring-shaped support member may be detachable from the workpiece mount allowing replacement thereof in the implanter, and the seal apparatus may further include a second seal member mounted on the support member with a radially outwardly extending rib adapted to engage a sidewall of the workpiece mount to provide a seal between the support member and the workpiece mount sidewall.
In accordance with still another aspect of the invention, there is provided a seal apparatus comprising a ring-shaped support member mounted on the workpiece mount, a first seal member mounted on the support member having a first seal portion flexible with respect to the support member and adapted to engage a portion of the backside of the workpiece proximate the peripheral edge to seal the front side of the workpiece from the cooling gas, and a second seal member mounted on the support member and having a radially outwardly extending rib adapted to engage a sidewall or surface of the workpiece mount to provide a seal between the support member and the workpiece mount sidewall.
According to another aspect of the invention, there is provided a batch ion implanter process disk pedestal for supporting a workpiece having a peripheral edge on a rotatable process disk. The pedestal comprises a workpiece mount adapted to support the workpiece in a plane, a gas feed port adapted to supply a cooling gas to a back side of the workpiece, a ring-shaped support member mounted on the workpiece mount, and a first seal member mounted on the support member having a first portion flexible with respect to the support member and adapted to engage a portion of the backside of the workpiece proximate the peripheral edge to seal the front side of the workpiece from the cooling gas. The ring-shaped support member may be detachable from the workpiece mount allowing replacement thereof in the pedestal.
The pedestal may further include a second seal member mounted on the support member and having a radially outwardly extending rib adapted to engage a sidewall or surface of the workpiece mount to provide a seal between the support member and the workpiece mount sidewall. The first portion of the first seal member may be flexible in a direction generally perpendicular to the plane of the workpiece, and cantilevered radially outwardly with respect to the center of the ring-shaped support member so as to pivot about a circumferential axis in a direction generally perpendicular to the plane of the workpiece.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.